SAIL-OFF provides 8 key pages for expert rigging, comprehensive maintenance tracking, and detailed sea trial logs, empowering every sailing adventure.

Reference & Training

Marine Weather Interpretation Guide

Learn to read synoptic charts, interpret barometer trends, identify cloud types, and understand sea-state forecasts — essential skills for safe passage planning.

Reading a Synoptic Chart

IsobarsLines of equal barometric pressure. Closely spaced = strong winds; widely spaced = light winds. Each line is typically 4 hPa apart.

High pressure (H)Descending air — clear skies, light winds, stable conditions. In the Southern Hemisphere, winds flow anticlockwise around a high.

Low pressure (L)Rising air — clouds, precipitation, stronger winds, unsettled conditions. Winds flow clockwise around a low (Southern Hemisphere).

Cold front (triangles)Cold air pushing under warm air. Brings squalls, wind shifts (typically backing SE→SW in NZ), temperature drop, and rain. Often preceded by cumulus/cumulonimbus.

Warm front (semicircles)Warm air sliding over cold air. Gradual cloud lowering, steady rain, visibility reduction. Wind shift is more gradual.

TroughAn elongated area of low pressure. Often brings increased cloud, showers, and wind shifts along the trough line.

RidgeAn elongated area of high pressure. Usually brings clearing conditions and lighter winds.

Pressure gradientThe rate of pressure change over distance. A tight gradient (isobars close together) means strong winds — the key indicator on a synoptic chart.

Barometer Trends

Steady then sudden drop

A low or front is approaching rapidly. Expect deteriorating conditions — increasing wind and possibly rain within hours. Consider seeking shelter or reefing early.

Steady rise

Conditions improving — high pressure building. Expect clearing skies and moderating winds.

Slow steady fall (1-2 hPa / 3 hrs)

A low is approaching gradually. Monitor forecast and prepare for changing conditions over the next 12-24 hours.

Rapid fall (3+ hPa / 3 hrs)

Storm or deep low approaching. Prepare for strong/gale conditions. Secure gear, reef down, or head for safe harbour.

Steady, no change

Current conditions likely to persist. Stable weather pattern.

Small diurnal fluctuation (±2-3 hPa)

Normal daily pressure variation due to atmospheric tides. Not a weather change indicator.

Cloud Types & What They Tell You

Cloud	Height	Appearance	Meaning
Cirrus (Ci)	High (6,000m+)	Thin, wispy, white hair-like streaks	Fair weather now, but may indicate a warm front approaching in 24-48 hours. Monitor barometer.
Cirrocumulus (Cc)	High	Small ripples or mackerel sky	Often precedes a change. "Mackerel sky, not 24 hours dry" — old sailors' saying.
Cirrostratus (Cs)	High	Thin milky veil, halo around sun/moon	Warm front approaching. Rain likely in 12-24 hours. Barometer will start falling.
Altocumulus (Ac)	Mid (2,000-6,000m)	Patches or rolls of grey/white	May indicate approaching instability. Monitor if thickening.
Altostratus (As)	Mid	Greyish veil, sun visible dimly	Warm front getting closer. Rain in 6-12 hours.
Cumulus (Cu)	Low (below 2,000m)	White cauliflower heaps, flat bases	Fair weather if small. If growing tall (towering), thunderstorms possible.
Cumulonimbus (Cb)	Low to high	Towering dark mass, anvil top	Thunderstorm. Expect heavy rain, squalls, lightning, strong gusts. Avoid or secure.
Stratus (St)	Low	Uniform grey layer, like elevated fog	Overcast, drizzle or light rain. Poor visibility. Light winds typical.
Nimbostratus (Ns)	Low to mid	Dark grey, thick, featureless	Steady moderate rain. Low visibility. Often associated with warm fronts.
Stratocumulus (Sc)	Low	Grey patches with some blue gaps	Fair to settled. Light rain possible but unlikely.

Sea State (Douglas Scale)

Calm (glassy)

< 1 kn

0 m

Calm (rippled)

1-3 kn

0-0.1 m

Smooth (wavelets)

4-6 kn

0.1-0.3 m

Slight

7-10 kn

0.3-0.6 m

Moderate

11-16 kn

0.6-1.5 m

Rough

17-21 kn

1.5-2.5 m

Very rough

22-27 kn

2.5-4 m

High

28-33 kn

4-6 m

Very high

34-40 kn

6-9 m

Phenomenal

40+ kn

9m+

Old Sailor's Wisdom

• "Red sky at night, sailor's delight — red sky at morning, sailor's warning"
• "Mackerel sky, not 24 hours dry"
• "When rain comes before wind, it's a warm front — when wind comes before rain, it's a cold front"
• "Long swell with no local wind — a storm is brewing far away"
• A falling barometer is nature's early warning — don't ignore it

This guide is for reference only. Always obtain official marine forecasts (MetService, PredictWind) before departing. Weather can change rapidly and without warning at sea.